1. Field of the Invention
The present invention relates to apparatus and method for forming an image (on a recording medium or for viewing) from an electrical signal and more particularly to improvements in electronic imaging apparatus and method of the kind using light valves.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,229,095 discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. Thus an array of such light valves comprises a panel of PLZT material with a plurality of interleaved electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing of such light valves in synchronization with the energization of red, green and blue exposing sources and according to the red, green and blue color information for the pixels of that image.
One preferred embodiment disclosed in the above-mentioned patent comprises a linear array of separately-addressable, light valve elements that are disposed in spaced transverse relation to the path along which a photosensitive imaging medium is fed. In this embodiment the pixels of the linear array are addressed concurrently, a line at a time, with image information and the movement of the imaging medium along its feed path is synchronized with successive actuations of the linear array. This embodiment of imaging apparatus is highly effective and advantageous in many applications; however, the requirement for relative movement between the image medium and linear light valve array gives rise to difficulties in certain applications.
Specifically, transverse banding artifacts (lines or streaks of different density extending transverse to the direction of relative motion between the imaging medium and linear array) sometimes occur because of improper synchronization between the electrical imaging signals and the physical motion between the image medium and imaging array. Such synchronization lapses, herein termed generally "flutter", can have many causes, e.g., imprecision of the movement of the imaging medium (such as can be caused by vibrations or "loose" tolerances) or variations in the frequency of the electrical signal (such as caused by imprecise movement of a recording medium on which the signal is stored). There are various synchronizing techniques to minimize these flutter artifacts; however, the remedies add to machine complexity and cost.
Also, imaging with linear arrays can give rise to longitudinal banding artifacts (lines or streaks of different density extending in the same direction as the relative motion between the linear array and the imaging medium) because of transmission non-uniformities between adjacent light valve elements in the linear array.
Further, although imaging with a linear light valve array is operative in a recording or printing mode, display imaging cannot be effected easily with a linear array.